Recent industrial advancement has required high purity and high quality of products, and thus a separator (or a membrane) technology has been recognized as a very important field. In the environmental sector, especially as the need for clean water and the awareness of a lack of water increases, a technology of using a membrane has largely attracted much attention as one of ways to solve these problems.
Processes such as water purification, sewage, waste water, and desalination using a membrane, are already spreading. In addition, the membrane technology has been developed for applications away from the membrane itself, and has expanded into surrounding technology development as well as has enhanced membrane performance improvement according to applications.
The membrane is a substance having a selection capability that is present between two different materials. In other words, the membrane means a substance which serves to selectively pass through or to exclude a material. Structures and substances of the membrane, and conditions and principles of the movement of the materials passing through the membrane, have no limitations. When a substance is located between only two materials to isolate the two materials each other, and the selective movement of the materials through the substance between the two materials occurs, the substance may be called a membrane.
The membranes are of a very variety of types and are classified into several criteria.
First, a classification by a separation operation is a classification method depending upon the state of a target material to be separated, and is classified into a liquid separation method, a gas-liquid separation method, a gas separation method, and so on. The liquid separation method is classified into micro filtration, ultra filtration, nano filtration, reverse osmosis filtration, etc., in accordance with the size of an object for filtration.
The gas separation method is classified in detail in accordance with the type of gas to be separated. The gas separation membrane is classified into an oxygen-enriched membrane for separating the oxygen gas, a nitrogen-enriched membrane for separating the nitrogen gas, a hydrogen-enriched membrane for separating the hydrogen gas, a dehumidifying film for removing humidity, etc.
The membrane is classified according to a film-like shape, and is classified into a flat membrane, a hollow fiber membrane, a tubular membrane, etc. In addition, the membrane is also classified into a plate-shaped type, a spiral wound type, a cartridge type, a flat membrane cell type, an immersion type, a tubular type, and so on, depending on the form of a filter module.
The membrane is classified according to a material and is classified into an inorganic film and an organic film using a polymer film. In recent years, however the inorganic films expand their use based on the advantages of heat resistance, durability, etc., most currently commercialized products are occupied by the polymer membranes.
In general, filtration means to remove two or more components from a fluid, that is, it means to separate undissolved particles (solid) from the fluid. Filtering mechanisms in the separation of the solid materials may be described as sieving, adsorption, dissolution, diffusion mechanisms. Except for some membranes such as gas separation membranes, reverse osmosis membranes, etc., it can be said that most of the filtering mechanisms depend entirely on the sieving mechanism.
Therefore, it is possible to use any materials with pores as filter media. Nonwoven fabrics (nonwovens), woven fabrics (wovens), meshes, porous membranes and the like are typical filter media.
It difficult to make pores not more than 1 μm in the case of nonwovens, wovens, meshes, etc. Thus, the nonwovens, wovens, meshes, etc., are used as a pretreatment filter concept with a limitation to a particle filtration area. Meanwhile, porous membranes can make precise and small pores and have been used for a process requiring a wide range of filtration areas and the highest precision such as micro filtration, ultra filtration, nano filtration, reverse osmosis filtration, etc.
Since the nonwovens, wovens, or meshes are made of fibers having a thickness from several micrometers to several hundreds of micrometers, it difficult to make fine pores not more than 1 μum. In particular, it is not possible actually to create uniform pores since webs are formed by random arrangement of fibers in the case of the nonwoven fabrics. The melt-blown nonwoven fabric may be called a nonwoven fabric made of a very fine fiber having a diameter of 1˜5 μm. The pore size before heat calendaring is not less than six micrometers and the pore size after heat calendaring is only three micrometers approximately. The deviation in the average pore size occurs more than ±20% around a reference point, and the melt-blown nonwoven fabric has a structure in which very large pores coexist.
Accordingly, the nonwovens, wovens, or meshes have the difficulty in preventing the leakage of contaminated materials through relatively large pores and thus have low filter efficiency. Therefore, the filter media are used in an inaccurate filtration process or used as a pre-treatment concept of an accurate filtration process.
Meanwhile, the porous membrane is prepared by a method such as a non-solvent induced phase separation (NIPS) process, a thermally induced phase separation (TIPS) process, a stretching process, a track etching process, a sol-gel process, etc. The materials of most of the porous membranes are made of representative organic polymers, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), nylon (Nylon6 or Nylon66), polysulfone (PS), polyethersulfone (PES), polypropylene (PP), polyethylene (PE), nitrocellulose (NC) or the like. While the conventional porous membranes may create pores of precise and small size, closed pores or blinded pores may be created inevitably in the manufacturing process. As a result, the conventional porous membranes have problems such as a small flow amount of filtration, a high driving pressure, and a short filtration lift cycle, to thus cause high operating costs and frequent filter replacement.
Korean Patent Application Publication No. 2013-0011192 discloses a method of producing a composite nonwoven fabric of alumina including a first step of performing a plasma treatment of a thermoplastic polymer fiber nonwoven fabric modify the surface of the nonwoven fabric, and a second step of depositing the alumina on the surface-treated fabric. However, the filter media using the alumina composite nonwoven fabric has no damage caused by cutting of the fiber, and is also excellent in the virus removal performance, but has a low filtering efficiency disadvantage due to the large pore size of the nonwoven fabric.